Cobalt chromium (CoCr), a well-known biocompatible material, was additively manufactured using direct energy deposition (DED) technology in this study. Since DED is a relatively new addition to additive manufacturing (AM) processes, there is not enough information about important properties of fabricated parts and components using this technology. This study investigates some important mechanical characteristics of the additively manufactured CoCr using a variety of numerical simulation methods in addition to mechanical tests and experiments. Mechanical experiments such as hardness, wear, and flexural bending test were conducted on DED processed samples. All experiments were also conducted on conventionally processed CoCr specimens for comparison purposes. This study attempts to explain mechanical properties in terms of microstructural characteristics of each sample. DED processed CoCr samples exhibited a complex microstructure with a variety of features such as cellular, columnar, and equiaxed grains within their melt pools. While the DED processed sample had a lower hardness compared to the conventionally processed one, it exhibited a higher wear resistance. These results were discussed in terms of microstructural characteristics and metallurgical bonding knowing that porosity level was negligible in both samples. The out-of-plane mechanical strength of CoCr samples was measured by conducting flexural bending test, and the conventional sample showed a higher flexural modulus than the DED sample. The bend tests were also numerically simulated using two different finite element analysis (FEA) procedures. The FEA results for the DED and conventionally processed samples follow the same trend as the results obtained from the experimental flexural bending test. The layer structure and interfacial bonding of the DED sample could have contributed to the lower flexural modulus compared to the conventional sample.

Among hardfacing processes using welding, laser cladding is nowadays one of the most efficient surface coating techniques. It is widely used to increase wear and corrosion resistance of machine parts as a result of the unique process characteristics such as low heat input (smaller heat affected zone), distortion free clad layers, lower dilution rate, finer coating microstructure as well as good metallurgical bonding at the coating/substrate interface. A wide range of new hardfacing materials and corrosion-resistant alloys are available on the market and in this study, different coatings of Ni-, Co- and Fe-based alloys as well as carbide-based metal matrix composites have been deposited by laser cladding for benchmarking purposes. Coatings were deposited onto mild steel substrates using a high-power diode laser. Coating microstructure and hardness were investigated as well as their tribological properties such as 2-body and 3-body abrasion, slurry abrasion and cavitation erosion resistance. Corrosion performance of coatings was also investigated with the salt spray test. Coatings are ranked according to their performance in the different tests and relationships between microstructure and coating properties are discussed.

The application of cold spray (CS) for additive manufacturing (CSAM) of structural components using metallic materials has recently attracted much attention. However, there are limited reports on developing thick deposits or components with high entropy alloys (HEAs) via CSAM and investigating the microstructural evolution and mechanical properties after deposition and subsequent annealing heat-treatment. This work investigated the microstructure and mechanical properties of asdeposited and heat-treated thick CoCrFeNiMn HEA deposit fabricated via CSAM. The microstructure of the HEA deposit and after heat-treatment were characterised using scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and x-ray diffraction (XRD). The microstructural analysis reveals heterogeneous grain size distribution with ultrafine grains at the particle-particle interfacial regions and coarse grains at the particle interiors in the as-deposited sample. The as-deposited sample, characterised by moderate porosity, was consolidated following the heat treatment at different temperatures. Additionally, increasing the temperature increases grain sizes resulting from static recovery and recrystallisation, with annealing twin formed at higher temperatures. Most notably, phase decomposition of the deposit microstructure occurs at 600 ºC, with Cr-rich phase particles formed at regions of high dislocations and grain boundaries. Nano-and micro-hardness and tensile testing of micro-flat dogbones samples were performed on the as-deposited and heattreated samples. The effect of heat-treatment on the microstructure and mechanical properties of the cold-sprayed HEA deposit were analysed and discussed.

In thermal spraying, one of the fundamental elements to achieving good bonding strength of the applied coating is surface preparation. Traditionally grit blasting using hard particles such as corundum is used to achieve suitable roughness on the substrate. Lately, there is an effort to find a suitable alternative from ecological and economical aspects. A promising possibility is laser texturing which enables the preparation of defined structures on the surface. Within a research project, procedures are developed to texture various substrates to direct application of HVOF coatings. The main goal is to achieve speeds of texturing comparable to grit blasting – more than 500 mm 2 /s while ensuring good bonding strength of the applied coating. This study focuses on HVOF spraying of Stellite 6 and WC-CoCr Coating. Selected substrates are steel, and then materials that cannot be traditionally grit blasted – nitrided steel and alumina ceramics. The study presents the analysis of laser textures on substrates, analysis of coating substrate-coating interface, and adhesion tests by tensile test. The most suitable textures – regarding the processing speed and achieved adhesion are selected.

The enhancement of the surface characteristics and corrosion resistance of cobalt alloys is under continuous examination for its biomedical applications. In this work, the investigation of corrosion performance of cobalt alloy coated with HA and HA/ZnO reinforced powders using plasma spray technique revealed that on the continuous increase of ZnO reinforcement the corrosion resistance improved progressively. The increment in surface hardness and drop in surface roughness was examined with the rise in ZnO content. Each coated sample exhibits a hydrophilic property. According to SEM and EDX investigations, homogeneous distribution of HA/ZnO coatings and intact reinforcement of ZnO in pure HA powder was noticed. All of the coated specimens maintain their morphological integrity, ensuring excellent protection of the prepared samples. The obtained outcomes denote HA/ZnO reinforced coatings on CoCr alloy as a suitable combination of enhanced surface properties and excellent corrosion resistance for future bone implant practices.

Impact testing appears as a most promising tool for gaining information on coating behavior in load-bearing applications. During dynamic impact test an indenter impacts successively the surface of the coating with constant force and frequency. The deformation of the coated specimen during impact testing is affected by the mechanical properties of both the substrate and the coating. Varying the impact load and the number of impacts, the evolution of coating surface deformation and contact fatigue failures can be observed. In the paper, the influence of dynamic impact load and number of impacts on the resulting impact crater volume and morphology is analysed, and the interpretation of the results in form of Wohler-like dependance is suggested and demonstrated on two types of HVOF sprayed Co-based alloy coatings. The low-number impact craters evolution and subsurface cracks propagation of HVOF sprayed Co-based alloy coatings is analyzed in more detail, by means of 3D optical microscopy and SEM. The results showed, that the higher ability to deform plastically increased the coatings dynamic impact fatigue lifetime. The cracks, responsible for coatings destruction, spread predominantly along the intersplat boundaries in the pile-up area.

A computational fluid dynamics model for understanding the HVAF process and the influence of the process parameters on the particle flight properties is investigated. Achieving this objective involves a novel approach to modeling the HVAF process with pressure inlet boundary conditions and integration of the mixing chamber. The study comprises the prediction of the flow fields described by a set of equations consisting of continuity, momentum, energy, and species transport. These equations are then solved with realizable k-ε turbulence model, a two-step chemistry model and eddy dissipation model to simulate the combustion reaction. Consequently, the interaction between the CoNiCrAlY alloy particles and the flow is modeled using a Lagrangian approach considering the forces acting on the particles and the heat transfer. The results show that the combustion chamber pressure is mainly affected by the compressed air and propane parameters. Furthermore, the flight behavior of the smaller particles is significantly influenced by the gas flow, while the larger particles tend to maintain their momentum and energy. Through the simulation model, an in-depth process understanding of the HVAF process can be achieved. More importantly, the model can be used as a tool for efficient process development.

As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have enabled the formation of a protective oxide layer and thus reduced the extent of internal oxidation. Due to the extensive oxidation ingress along the large Laves phase, it is concluded that T14 is not suitable for applications in SHS at 800 °C.

This study evaluates the hot corrosion behavior of NiCoCrAlY, NiCoCrAlYHfSi, NiCoCrAlTaReY, and CoCrAlYTaCSi coatings on 1.4923 stainless steel, applied by high-pressure HVOF spraying. All coatings were cycled in in an environment of Na 2 SO 4 and 82% Fe 2 (SO 4 ) 3 at 690 °C. Each cycle consisted of 1 h of heating in a silicon carbide tube furnace followed by 20 min of cooling. Weight change measurements were performed after each cycle to track corrosion kinetics, and SEM and EDS analysis were employed to analyze the corrosion mechanism. CoCrAlYTaCSi showed the best corrosion resistance of the coatings tested.

This work investigates the high-temperature oxidation kinetics of CoCrAlSiY coatings with different Si concentrations. Hot-corrosion resistance is determined at 800 and 900 °C via hot salt coating, thermal shock resistance is measured at 1050 °C, and the oxidation and corrosion products are analyzed through mineralogical and micro analysis. The results show that Si promotes the formation of an Al 2 O 3 film that improves oxidation and corrosion resistance, but excessive amounts reduce thermal shock resistance.

Due to their superior wear resistance and oxidation behavior, Stellite coatings are widely used in industrial applications where they are exposed to high temperature. Common processes for applying Stellite coatings include high-velocity oxyfuel spraying, laser cladding, and plasma transferred arc welding. Although Stellite welding consumables are available, there are few studies on arc-sprayed Stellite coatings in the literature. This work investigates the microstructural characteristics of an arc-sprayed deposit produced using a CoCr-based cored wire with 4.5 wt% W. The deposit is examined both in its as-sprayed state and after high-temperature exposure. Microstructure formation is assessed via SEM and EDX analysis, phase transformation processes are determined by XRD analysis, and friction and wear properties are measured. The findings are presented and discussed and compared with those obtained from conventional CoCr-based coatings.

Ceramic coatings are often applied to metallic substrates using a bond coat to promote adhesion. The corrosion environment between the substrate and ceramic layer can be very harsh due to the absence of dissolved oxygen, high concentrations of corrosive electrolytes, and galvanic and crevice corrosion mechanisms within the bond coat itself. This study assesses the performance of several bond coats in sulfuric acid, including plasma sprayed tantalum and HVOF sprayed NiCr, Hastelloy C, and Ultimet. The bond coats were deposited on Hastelloy substrates and covered with a Cr 2 O 3 topcoat. Electrochemical polarization and open circuit potential measurements were recorded and coating microstructures were examined before and after the corrosion tests. Plasma sprayed tantalum exhibited the best corrosion resistance at room temperature, followed by HVOF sprayed Hastelloy C.

This paper evaluates the performance of a new single-cathode cascaded arc spray gun developed for low-pressure plasma spraying (LPPS). It describes key design features, explaining how they contribute to arc and voltage stability, improved thermal efficiency, higher throughput, and extended equipment life. It assesses the effect of nozzle geometry on spray spot morphology and examines the microstructure of CoNiCrAlY and YSZ coatings deposited on different substrates, including a turbine blade, using the new gun. Cross-sectional images show the uniformity of the CoNiCrAlY coatings in different locations on the turbine blade, including platform, fillet, and airfoil surface.

The plasma torch design affects the particle-plasma interaction, in-flight properties and the coating microstructure. When spraying metallic powders, the in-flight oxidation as well as the particle velocity and temperature determine the mechanical, corrosion and oxidation properties, which have a major impact on the in-service degradation of bond coats. This study aims to determine the microstructural and mechanical properties of as-sprayed CoNiCrAlY coatings deposited on the Inconel 718 alloy. Depositions were made using a High Velocity Plasma Spray Process (HVPS), which is based on a special plasma torch design. In-flight particle characteristics were determined to elucidate the kinetic and thermal regime of HVPS process.

The erosion behaviour at room temperature (RT) of as-deposited SPS, EB-PVD and APS YSZ-based TBCs was investigated. All coatings were deposited on Inconel 625 alloy coupons. The same APS CoNiCrAlY bond coat was employed for all SPS and APS TBCs. The erodent material was 50 μm alumina and the impact angles were 15° and 90°. A total of 4 different types of SPS YSZ-based TBCs were tested, which consisted of two distinct columnar-segmented and two distinct columnar-grown microstructures. The EB-PVD and APS YSZ TBCs were employed as benchmarks. The erosion performance of the different TBCs in this study was ranked based on the coating volume loss after wear testing. The TBC microstructures and phase compositions were evaluated via SEM and XRD. The erosion mechanisms of the different TBCs were compared by analyzing the cross-sectional and top surface microstructures of the as-sprayed and eroded TBCs. These are released results from the Surftec Industrial R&D Group of the National Research Council of Canada (NRC).

In this study, the mechanisms responsible for enhancing the adhesion strength of thermally sprayed metallic coatings subjected to vacuum heat treatment were investigated using atmospheric plasma sprayed (APS) CoNiCrAlY coatings as an example. The formation of metallurgical bonding between the coating and the substrate, which determined the increase in the adhesion strength of the coatings, was studied by analyzing the effect of morphological changes of the oxide film in the coating. The results showed that during the vacuum heat treatment process, the oxide film formed during the coating deposition gradually broke down and subsequently shrank into round oxides. After vacuum heat treatment, the adhesion strength of the coating improved significantly, and there was a positive nonlinear relationship between the treatment time and the adhesion strength. The increase in the adhesion strength was caused by the formation of metallurgical bonding between the coating and the substrate. However, the prerequisite for the formation of metallurgical bonding was that the oxide film had to break during the vacuum heat treatment process. A thermodynamic 2D model based on the thermal grooving theory was proposed to explore the essential conditions for the breaking and shrinking of the oxide film.

Corrosion resistance of coatings deposited by thermal spraying technology HVOF (High Velocity Oxygen Fuel) requires high density in coating and good adhesion to substrate material. The majority of thermally sprayed materials meet the requirements of high corrosion resistance in terms of their composition. However, porous structure raises doubts about the performance of thermally sprayed coatings regarding sufficient protection to the base material. In fact, corrosion protection is a basic coating function. However, , no sufficient attention has been dedicated to the issue of component protection against corrosion attack using HVOF sprayed coatings. In this study, NiCoCrAlY, NiCoCrAlTaReY, NiCoCrAlYHfSi, and CoCrAlYTaCSi coatings were deposited on the substrate material 1.4923. The coatings were deposited using HP/HVOF (High Pressure / High Velocity Oxygen Fuel) thermal spraying technology. The coatings were exposed to the corrosive-aggressive environment in the form of molten salts mixture with composition of 60 % V 2 O 5 and 40 % Na 2 SO 4 at the selected temperature of 750 °C. Further, all coatings were exposed to cyclic conditions. Weight changes of individual specimens were measured after every cycle and results were recorded in diagrams. After the corrosion test, all evaluated coatings were analyzed using scanning electron microscope (SEM), analysis of elemental composition (EDS) and X-Ray diffraction. The NiCoCrAlY and NiCoCrAlTaReY coatings showed the best corrosion protection in selected corrosive aggressive environment, forming the protective oxide layer that prevented further corrosion attack. On the contrary, NiCoCrAlYHfSi and CoCrAlYTaCSi coatings were found not to be suitable for corrosion protection of components working in selected corrosive environment.

The study of both the interface strength and residual stresses within a plasma sprayed ceramic coating is of great interest which main purpose is a better understanding of the mechanical properties of metal/ceramic systems. In this work, experiments involving a LASAT facility (LASAT: Laser Shock adhesion test) were implemented in order to analyse the adhesion and the damaging behaviour (debonding and buckling) of alumina coatings onto Co-based alloy. Similar alumina coatings were deposited using same plasma parameters with various surface preparations: smooth or severe grit blasting, with and without pre-oxidation. The non-destructive analyses (Optical and IR imaging) of the buckled region after LASAT have allowed to compare and discuss the interface strength of the studied coated samples. Further discussion was carried out by analysing the blister, resulting from the release of residual stresses within the coating after LASAT. It was thus evidenced that the residual stress state is a key parameter on resulting adhesive properties. This explorating work suggests using the LASAT method to analyse the adhesion and residual stresses within thermal sprayings.

A Vickers indentation method has been applied to determine the interfacial fracture toughness of modern multilayer thermal barrier coatings. The delamination behavior of four types of coating systems will be discussed and compared with results based on modified four-point bending (4PB) tests. The investigated multi-layer coating system consists of a CoNiCrAlY-bond coat applied via low-pressure plasma spray (LPPS) on a nickel-based superalloy and an atmospheric-plasma sprayed (APS) top layer of type gadolinium zirconate (GZO) and yttria-stabilized zirconia (YSZ). A conventional YSZ mono-layer system is used for reference. The effects of GZO and YSZ microstructure were investigated using top coats with low and high porosities for both (multi- and single-layer) coating systems. Isothermal oxidation tests at 1100 °C up to 500 hours were performed to study the interaction between thermal aging and fracture behavior. Investigations of microstructure and sintering behavior show a significant influence of the annealing conditions on fracture toughness. It has been observed, that with increasing annealing time, the stiffness and thus the crack driving force of the GZO layer is increased due to sintering effects and healing of submicron defects. The lower stiffness and higher defect density of GZO seem to be the main reason for the reduced fracture toughness of the YSZ / GZO interface compared to the YSZ / CoNiCrAlY interface. As a result, the delamination of the top coat is observed to shift from the top coat / bond coat interface into the top coat double-layer.

Due to good performance in abrasive and sliding wear and enhanced oxidation behavior, coatings based on Co-Cr-W alloys are widely used in industrial applications, where the material is exposed to high temperature. Within the scope of this study, a Co-based alloy similar to commercial Stellite 6, which additionally contains 20.6 wt.% of vanadium, was deposited by Twin Wire Arc Spraying (TWAS). Multi-criteria optimization using statistical design of experiments (DoE) have been carried out in order to produce adequate coatings. The produced coatings have been analyzed with respect to their tribological behavior at elevated temperatures. Dry sliding experiments were performed in the temperature range between 25°C and 750°C. Oxide phases were identified in the investigated temperature range by X-ray diffraction (XRD) using synchrotron radiation. The V-doped Stellite-based coating possesses a reduced coefficient of friction (COF) of about 0.37 at elevated temperatures (above 650°C), which was significant lower when compared to conventional Stellite 6 coating that serves as reference. In contrast, both produced coatings feature a similar COF under room temperature. X-ray diffraction reveals the formation of cobalt vanadate and vanadium oxides above 650°C. The formation of vanadium oxides exhibits the ability of self-lubricating behavior, thus leading to enhanced tribological properties.